This invention relates to the process for electrolytic treatment of aqueous alkali halide solution using of ion exchange membranes, and more specifically, it relates to the above kind of process by use of a electrolytic bath assembly comprising an anodic chamber, an intermediate chamber and a cathodic chamber arranged in series each chamber being separated by an ion exchange membrane from its adjoining chamber.
The process for the electrolysis of aqueous alkali halide solution in a bath assembly fitted with selectively operating ion exchange membranes has hitherto been known. More recently, with remarkable development of fluorine containing resin-made, ion exchange membranes, this process has been practised on the industrial scale.
It has been experienced, however, that the current efficiency is appreciably reduced with increase of alkali concentration in the cathodic chamber, when an ion exchange membrane of the normally procurable kind and type is used. This is especially true, even with use of rather superior membranes providing as high as 80% or higher current efficiency at 20% or lower alkali concentration in the cathodic chamber, this current efficiency frequently becoming 50% or less when the alkali concentration increases to 30% or higher. It has already been proposed to provide an intermediate chamber between the anodic and cathodic chambers in order to avoid the above-mentioned disadvantage resulting from the increased alkali concentration in the cathodic chamber, so as to separate these chambers one after another by the provision of respective ion exchange membranes for the purpose of keeping the difference in the alkali hydroxide concentrations between the neighboring chambers as little as possible, as would necessary for the avoidance of current efficiency reduction. The number of such intermediate chambers may not be limited to only one, thus, it may be increased as occasion desires.
With increased number of such intermediate chambers for obtaining as little as possible difference in the concentrations in these chambers for improving the current efficiency, the number of the ion exchange membranes separating these chambers one from another would correspondingly, be increased, with increased interelectrode distances, thereby requiring a correspondingly increased voltage to be impressed.
It is, therefore, highly desirous to minimize the number of the intermediate chambers as much as possible it being of still greater advantage to operate the process with a three chamber mode of the electrolytic bath assembly, the number of the intermediate chambers being thus limited to only one, thereby to obtain an operating current efficiency as high as possible.
We have extensively investigated, in order to realize a high efficiency process for obtaining high concentration aqueous caustic alkali solution with high current efficiency, the use of the three chamber mode of the electrolytic bath assembly comprising a cathodic, an intermediate, and an anodic chamber arranged in series and fitted with ion exchange membranes acting as the separating walls between the chambers.
It may be stressed that the membrane acting as the separating wall between the anodic and the intermediate chamber must be highly durable to gaseous chlorine and caustic alkali present respectively in the anodic and the intermediate chamber. A most preferable one serving for this purpose is a fluorine-containing resin-made anodic ion exchange membrane. As results with the conventional two-chamber system, the current efficiency will appreciably decrease with increase of the caustic alkali concentration in the intermediate chamber which is comparable to the cathodic chamber in the known conventional system, and thus, the alkali concentration in the intermediate one should preferably be lower than 20%.
As the cationic permselective membrane acting as the separating wall between the intermediate chamber and the cathodic one may be made of any suitable substance if it is durable to caustic alkali. But, in practice, it must have a low electric resistance in aqueous caustic alkali solution and a high selective penetration performance to Na.sup.+. Further, this membrane must have a moderate water penetration performance, since if an excess quantity of water should transfer from the intermediate chamber through the membrane to the cathodic chamber, the alkali concentration prevailing in the latter will be disadvantageously reduced.
As, conventionally, when all the quantity of caustic alkali formed in the intermediate chamber is transferred to the cathodic chamber of the three bath chamber system, the alkali concentration in the intermediate chamber must become disadvantageously high, when high concentration caustic alkali is to be attained at the cathodic chamber. In such case as above, the current efficiency will become highly inferior and under extreme conditions, the electrolytic apparatus does not operate properly, the operational troubles being caused by the too much high alkali concentration near the condensation state.
We propose, therefore, to separately take out the caustic alkali electrolytically formed in the intermediate chamber, or to transfer part or the whole thereof to the cathodic chamber so as to produce high concentration caustic alkali at the latter chamber, as an alternate measure.
According to our experiments, it has been found that in order to provide high concentration caustic alkali by high current and power efficiency with use of the three chamber type bath assembly with ion exchange membranes, the anodic ion exchange one acting as the separating wall between the intermediate chamber and the cathodic one, must have lower penetration performance to water as well as hydroxide radicals, and an especially small electric resistance value in the caustic alkali solution. For this purpose, we have found that those having phenol radicals acting as the ion exchange one are highly suitable.